To study chaotic phenomen in unstable systems, we excited two types of instabilities by injecting an electron beam or passing a current into a Double plasma. Then, we adjusted to make the system chaotic by controlling the growth rate of the instabilities. The experiments were performed using a Double-Plasma Device. Argon gas was used and the pressure was ranged from 10^<-4> to 3*10^<-4> Torr. The Plasma parameters were : n_e= (1-5) *10^8cm_<-3> and Te=1eV.Obtained main results were as follows :[1] Electron beam plasma system :(1) By injecting an elecron beam whose energy is 10-100eV into the plasma, we excited the electron beam plasma instability whose frequency is around 200MHz.(2) When the electron beam density was increased, the period doubling appeared. The period-2 and period-3's bifurcation took place, showing that there is a window.(3) We calcurated the correlation dimension D by the embeding method and found that D=2.5 for period-3 and 3.3 for period-4, respectively.(4) The max
… Moreimum Lyapunov exponent was positive, suggesting that the system is chaotic.(5) It is concluded from (3) (4) that the electron beam plasma system is in the chaotic state.[2] Ion acoustic instability system :(1) In order to pass a current in the plasma, we installed two grid meshes in the plasma and applied DC potentials to one of the grids. When the current exceeded a certain value, the ion acouistic instability was excited, where the upper cut off frequency was around the ion plasma frequency. The electron drift velocity was measured with the probe and amounted to 40Cs (Cs : ion acoustic velocity).(2) Above results are understood from the dispersion relation of the current-driven ion acoustic instability.(3) Observed time series showed that the system becomes intermittent chaos above a threshold. The correlation dimension was non-integer and larger than 2. The maximum Lyapunov exponent was positive, which suggests that the system is in an intemittent chaotic state.(4) The spectrum took a form of f^<-1>, so that observed intemittency is Type-1. Less